Because he took all proper precautions to avoid damaging the line. I'm still looking into the specific wiring codes but my understanding is that wiring installation is required to be done in such a way that it's not easily damaged and so that further repairs can be made, such as by securing it in a conduit or using cable clamps to ensure the line isn't pulled by its own weight.

If their initial install was not up to code, then they are liable for the repairs, but since their contract is with the tenant and not myself, I didn't know if I had standing to sue them directly. Their neglect led to a monetary loss for me, even though it was indirect.